Fuel pumps used to supply fuel to reciprocating aircraft engines are driven through suitable drive couplings between the pumping element and associated gear boxes inside the engines.
Because this type of pump is a positive displacement pump, the fuel output of the pump is usually greater than the fuel flow requirements of the engine or aircraft system downstream of the pump. Any excess flow is typically routed from the high pressure side of the pump back to the low pressure side through an integral bypass valve.
To prevent overboard leakage of fuel between the pump housing and mating valve housing, a flat elastomeric gasket is conventionally used to provide a seal at the pump housing/valve housing interface. In addition, a diaphragm seal is conventionally clamped between the valve housing and associated valve cover to prevent leakage at this interface.
Although such seal arrangements have generally performed quite satisfactorily over the years, their ability to prevent overboard leakage of fuel under all operating conditions is sensitive to assembly technique, compression set resistance of the elastomeric gasket and diaphragm seal, and operating temperatures. The gasket and diaphragm seal are susceptible to compression set when exposed to high thermal stresses with a corresponding reduction in the compressive pre-load on the gasket and diaphragm seal once the thermal stresses are relieved, resulting in a greater potential for a fuel leak path through the clamped joints.